1. Field of the Invention
This invention relates generally to the bulk measurement of trace contaminants in the surface layers of semiconductor wafers and dies, as well as material composition as a function of depth. More particularly, the invention pertains to improvements in methods and apparatus for mass spectrographic analysis of wafer and semiconductor die surface layers.
2. State of the Art
Secondary ion mass spectrometry (SIMS) is known as a method for determining particular constituents of a semiconductor material and providing a quantitative measurement of each.
Generally, this method involves bombarding a sample with "primary" ions e.g. oxygen ions, measuring the intensities of secondary ions emitted or sputtered from the sample, and calculating the quantity of each conductive impurity based on the secondary emission as compared to the emission of standard materials. The sputtering and analysis is typically conducted in an ultra-high-vacuum environment.
SIMS may be used to achieve parts-per-billion (ppb) detection limits for bulk analysis and for determining material composition as a function of depth, provided the sample size is sufficiently large. The extreme sensitivity of SIMS results from its ability to "consume" large amounts of sample material, and thus process a large number of atoms to detect. However, because of the high rate of material consumption from a very small sample, dynamic SIMS is generally not appropriate for analysis of a very thin oxide surface layer of a semiconductor die and plurality of semiconductor dice in wafer form. Typical semiconductor contaminants may include lithium, boron, sodium, potassium, iron, sulfur, and carbon, all of which are found in the oxide layer on the semiconductor die surface. For the case of surface contaminants on silicon, the oxide layer is generally not more than about 15 .ANG. thick. However, several minutes are required to obtain a sufficient number of data points at the required analyte masses, so the method is not useful for this application as the oxide layer will be quickly consumed.
It is desirable to be able to detect the concentrations of boron, lithium and sodium to less than about 1.times.10.sup.6 atoms per square centimeter of semiconductor die surface area. These detection limits are considerably lower than currently obtainable.
U.S. Pat. No. 4,874,946 of Kazmerski discloses a method and apparatus for mapping the chemical composition of a solid device, using a rasterable SIMS mass analyzer.
U.S. Pat. No. 4,611,120 of Bancroft et al. discloses a method for suppressing molecular ions in the secondary ion mass spectra of a commercial SIMS instrument.
U.S. Pat. No. 5,521,377 of Kataoka et al. discloses a method for analysis of a solid in a planar or depth-wise direction using sputtering with two ionizing beams and detecting a two-atom composite ion.
U.S. Pat. Nos. 5,502,305 of Kataoka and 5,442,174 of Kataoka et al. disclose methods for analysis of a solid in a planar or depth-wise direction using sputtering with an ionizing beam and detecting a three-atom composite ion.
U.S. Pat. No. 5,332,879 of Radhakrishnan et al. discloses the use of a pulsed laser beam to remove contaminant metals from the surface of a polyimide layer. The disclosure indicated high surface metal removal with "minimal" removal of the polyimide, i.e. 250-500 .ANG. per pulse. Such ablation rates are far greater than useful in the analysis of surface contaminants in semiconductor devices, where the surface oxide layer is typically only about 15 .ANG. in depth.
Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has also been found useful for bulk analysis of materials, provided the sample size is sufficiently large. The TOF-SIMS instrument directly measures the speeds of secondary ions by measuring the time taken to travel a given distance. Knowing the ion's energy, which is defined by the spectrometer's acceleration voltage, its mass can then be calculated. Typically, the time intervals are defined as the difference in time between pulsing the ion gun and the ion arrival at the detector. The mass range is then calibrated using at least three known mass peaks.
TOF-SIMS instruments have been found to provide some of the lowest detection limits in surface analysis, typically even lower than total reflection X-ray fluorescence (TXRF) with vapor phase decomposition (VPD). For the TOF-SIMS instrument, some representative detection limits are &lt;1.times.10.sup.8 atoms/cm.sup.2 for lithium, boron and sodium, and &lt;1.times.10.sup.9 atoms/cm.sup.2 for iron.
The TXRF instrument, on the other hand, is incapable of detecting elements lighter than sulfur, so critical elements such as sodium, carbon, lithium and boron cannot be detected.
Thus, the TOF-SIMS method would appear to be potentially useful for surface analysis, but instrumental constraints limit the sampling area to about 100.times.100 .mu.m, and sampling of a relatively shallow oxide layer over the 100.times.100 .mu.m area does not produce sufficient sample material for achieving the desired detection limits.
For TOF-SIMS, the detection limits are determined by the transmission and exceptance of the mass spectrometer, the sputter and ionization yield of the analyte, and the amount of material consumed during the analysis. These parameters may be categorized as the useful yield of the mass spectrometer and volume of analyte. Sampling of the maximum raster area of 100.times.100 .mu.m to a depth of about 13 .ANG. will produce about 3.times.10.sup.11 particles. This is equivalent to between 0.3 to 30 (thirty) counts of a measured component at the 1 ppm level depending upon the ionization yields. It is critical to semiconductor device manufacture that bulk concentrations of some contaminants as low as 0.01 ppm and even 1 ppb be accurately detectable. Thus, current detection limits for certain contaminants must be reduced by a factor on the order of about 100 or more.
U.S. Pat. No. 5,087,815 of Schultz et al. discloses a method and apparatus for a TOF-SIMS isotopic ratio determination of elements on a surface.